Unison notes of IDENTICAL duration across two voices: the majority default is ONE shared notehead with opposed stems — Gradus should deliberately override that default, not accidentally fall short of it

When two voices sharing a stave sound the identical pitch with the identical duration, the convention used by Gould as the primary recommendation, and implemented as the automatic default in Dorico, Finale, and Sibelius alike, is a SINGLE shared notehead carrying two stems, one up and one down ('double-stemmed unison') — the two voices are understood to coincide for that instant and the reader is not asked to parse two overlapping heads. A secondary, less common convention — used by some house styles, and by Ross as an alternative worth naming, specifically WHEN the engraver wants the two independent parts to stay visually traceable even through a momentary unison (e.g. a species-counterpoint or chorale exercise where the point of the exercise is seeing two lines) — keeps two separate, very slightly offset (canted) noteheads instead.

Gould, Behind Bars: the double-stemmed single notehead is presented as the standard resolution for a same-duration unison, and is also the universal software default (Dorico, Finale, Sibelius all merge automatically when duration and pitch match). Ross, The Art of Music Engraving, documents the canted separate-notehead alternative for contexts where voice-independence must stay visible. The PRIOR draft of this rule had this backwards — it named separate noteheads as 'Gould's stated default,' which overstates the case: Gould presents the shared notehead as the default and the separate-notehead option as the exception for cases wanting extra clarity.

This rule needs the notated content and the rendered result together — the score says what should happen, the page says whether it did.